Monday, January 14th, 2008 | Popularity: 40%
Dear, Andrew. My sister was an evacuee in the major San Diego Fire. She lives in Rancho
Bernardo. She is an area coordinator for neighbor hood watch. At the last meeting after the fire an attending fire captain explained an interesting phenomena many of the homes that were destroyed and had tile roofs had something happen to them that most other homes did not.
The fires started in the middle of the building not the out side. After a through investigation the fire dept. came up with two causes. The first was that homes that had curved tile roofs and did not have bird blocks embers blew between the tiles and started the roofs on fire under the tiles. The second cause was that older tile roof home had attic vents that had 1/4 ” mesh screen or larger and the embers just blew through the screens into the attic causing the fires. When the winds came through the Rancho Bernardo area they were around 40 to 60 mile an hour. It was like a blast furnace and temperatures in some areas exceeded 2000 degrees. With the wind blowing in a horizontal direction the heat and the winds blew embers and extremely hot air into the attics of the homes. The fire dept. is now recommending that new homes and older ones be fitted with attic vents of 1/8 ” to prevent small embers from being blown into the attics. The only problem here is that the mesh being extremely thin my also melt or burn at lower temperatures. You may want to pass this information on to Straw Bale Builders and run it
by your local fire dept.
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Here is a very interesting email I recently received. It is definitely worth a look. I live in the country and am quite aware of the places fires start: leaves under the deck, debris in foundation vents, trees not properly maintained near the house, etc. I had not heard of the problems discussed in the following email although I clearly see how they could be a problem. If you live in fire prone areas, be sure to read on.
Dear, Andrew. My sister was an evacuee in the major San Diego Fire. She lives in Rancho
Bernardo. She is an area coordinator for neighbor hood watch. At the last meeting after the fire an attending fire captain explained an interesting phenomena many of the homes that were destroyed and had tile roofs had something happen to them that most other homes did not.
The fires started in the middle of the building not the out side. After a through investigation the fire dept. came up with two causes. The first was that homes that had curved tile roofs and did not have bird blocks embers blew between the tiles and started the roofs on fire under the tiles. The second cause was that older tile roof home had attic vents that had 1/4 ” mesh screen or larger and the embers just blew through the screens into the attic causing the fires. When the winds came through the Rancho Bernardo area they were around 40 to 60 mile an hour. It was like a blast furnace and temperatures in some areas exceeded 2000 degrees. With the wind blowing in a horizontal direction the heat and the winds blew embers and extremely hot air into the attics of the homes. The fire dept. is now recommending that new homes and older ones be fitted with attic vents of 1/8 ” to prevent small embers from being blown into the attics. The only problem here is that the mesh being extremely thin my also melt or burn at lower temperatures. You may want to pass this information on to Straw Bale Builders and run it
by your local fire dept.
Popularity: 40%
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Andrew Morison is a licensed contractor specializing in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs. You can check these out at http://www.LearnStrawBale.com.
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January 16th, 2008 at 9:43 am
Andrew: Very much appreciated this latest article on attic vents/wildfires.
Wouldn’t some kind of metal louvres block the embers coming through the wider wiremesh screens, wider mesh being more efficient for actual ventilation of moving air.
Could metal louvres (installed as a backup to the wire mesh coverings) be controlled by an electric switch mechanism somewhere in the house below for quick manipulation of closing louvres in an emergency situation?
Could someone explain what “birdblocks” are? Would that be something to keep birds from nesting in the open rooftile ends??
January 16th, 2008 at 1:11 pm
Attic vents have been known to be a serious wildfire problem for a long time. The Wildland-Urban Interface Code has prohibited soffit vents for years since hot gasses tend to concentrate beneath soffits and then circulate up through the vents and into the roof structure causing ignition. Soffit vents are now illegal in California and some other jurisdictions. There’s no good workaround that I know of. As a result, some experts now recommend unvented roofs, which of course introduces additional complexities to roof design.
January 17th, 2008 at 10:39 am
Andrew,
our new (Straw Bale) house in California was NOT ALLOWED to have soffit vents. We had to change the plans for low roof vents along the edge. The strange part is the house next door was built with sofit vents. The only reason I can think of is that their plans were approved in the prior year, maybe before the regulation took affect?
January 17th, 2008 at 4:17 pm
Why are we considering vents as merely a hole cut with some sort of covering? If you want to vent an attic in a fire prone area, who says the vent cant be ducted… from the ground up; or perhaps be channeled with a couple of 90 degree turns before it opens into the attic? True convection will take place if we properly duct or plan a diversion pathway into and out of the attic… it is merely a function of hot air rising and cold air then filling the vacuum created. I am sure you could find a venting method if you would think along the lines of hvac ducting, only external.
January 18th, 2008 at 8:02 am
I think this is a special case when Santa Ana winds are present. If you don’t live in an area where 70 to 100 mph winds drive wildfires as they do in southern california(I am from Poway but live in Northern Arizona), I don’t think it is as much an issue. The winds are what pushed the embers into the attic, not the nature of wildfire itself.
January 22nd, 2008 at 2:32 pm
Hey, this is an interesting conversation. I’m just surprised it’s news to so many on the list.
Adrien’s hunch is incorrect — the problem is an inherent aspect of most wild fires, and, in fact, Arizona jurisdictions (see Pima County website, for example) have the same prohibition as California on soffit vents. A wild fire creates its own wind, which drives embers forward of the fire front by long distances. These can ignite a building a half mile or so from the main fire.
Jared’s idea of providing an alternative cool air source from ground level is one that I thought of also (running the stacks from the crawlspace up through the bale wall), but when I ran it past an expert was told the “stack effect” of having a long “chimney” bringing the air up from the ground would likely make the situation even worse, and might even suck fire into the crawlspace as well. Anyway, since the air has to exit the roof someplace, this vent air arrangement still doesn’t deal with the likelihood of embers getting into the roof structure through those openings — be they at the ridge or at the dormer ends.
Are there any experts out there who can help with this?
January 23rd, 2008 at 9:57 pm
Great discussion!
Fine Homebuilding magazine published an article about designing for wild fires in their April-May 2004 issue that addresses some of these questions. Authors Stephen Mead and Jim Wheeler recommended locating soffit vents in the facia instead of the blocking under the eves, theory being that hot gasses and embers wouldn’t be circulating and trapped there. Bob Theis, an architect and active member of the California Straw Building Association (www.strawbuilding.org) has been suggesting that detail for a long time now, using cement board and/or plastering the underside of the soffits/eves so hot gasses and embers don’t have anything to ignite as they swirl around under the eve.
Because gutters occupy most our facia boards we’re installing soffit vents directly behind the facia board, and running cement board right up to the junction of the box beam over the straw bale wall. We also used 1/8″ screens…something recommended in a series on wild land fires that appeared in our local newspaper, the Applegater, a few years ago. I’ve seen people use corrugated steel instead of cement board, too.
Jim Reiland
Jacksonville, OR
January 27th, 2008 at 9:17 pm
How about using structual integrated panels and have vaulted ceilings. The couple I’ve seen don’t have sofit venting at all.
http://www.sips.org/
January 28th, 2008 at 9:09 am
Brian,
Yes. SIPs are a great idea and like you say, do not require venting. Another option is to use spray foam insulation which also does not require ventilation. There is actually a spray foam insulation that is water blown and soy based which is therefore quite benign. You can find out more at www.biobased.net.